Recent advancements in semiconductor packaging have led to the development of the “stacked” package, in which two or more semiconductor dies are mounted on top of one another within a single semiconductor package. This stacking of dies enables increased functionality in a small footprint, allowing for downsizing of the overall semiconductor package. Typically, an adhesive paste or film is used between the two semiconductor dies to ensure package integrity during fabrication operations such as wirebonding, molding, and solder reflow, and during end use.
There are various methods of assembling a package in a stacked configuration. Each die contains a number of electrical terminals, from which metal, usually gold, wires extend to electrical terminals on a substrate. In a stacked package the wire bonds from one die must avoid contact with and damage to the neighboring dies. Furthermore, the adhesive used to bond the dies to the substrate and/or to one another must not encroach on the wirebonding pads. As packages migrate to tighter tolerances in general, and in particular in stacked packages, there is a need for very tight control of the adhesive fillet, which typically protrudes from the edge of the bonded die.
One method that has been used to control the flow of the bonding adhesive is to utilize a tape or film instead of a paste adhesive. In this method the tape or film adhesive is applied to either the substrate or one of the dies. When applied to the dies, the adhesive film can be attached first to the wafer, which is then singulated into dies, or it can be applied directly to previously singulated dies. The die is then attached to its substrate and bonded through the use of heat and pressure. This provides the required flow characteristics for the stacked package, with minimal fillet formation around the periphery of the die. However, tapes and films are very expensive compared to traditional paste die attach adhesives and it would be preferable to utilize a paste, or liquid type adhesive that could be applied to the wafer and then partially cured or dried (B-staged) to enable further processing.
One known method of coating a wafer is spin coating. This method has the advantage of speed and has been successfully utilized with adhesives containing minimal filler, typically less than 10% by weight. Higher filler loadings are desirable, however, for a number of purposes, including stress reduction, control of modulus or coefficient of thermal expansion, reduced moisture absorption, reinforcement of the material by improving cohesive strength, electrical and/or thermal conductivity, reduced volatility, and flow control. Although useful for these functions, higher filler loadings can lead to a loss in uniformity of coating thickness, resulting in a thicker coating in the center of the wafer as compared to the outer edges. This lack of uniformity in coating thickness is undesirable for current subsequent processing steps including dicing and die attach.
This invention provides a solution to the above problem by providing a semiconductor wafer that is coated with a filled, spin-coatable material that has good uniformity of coating thickness.